The use of appropriately labeled antibodies for tumor location has been suggested in the literature for many years (D. Pressman and G. Keighley, J. Immunology 59, 141 (1948); reviewed in D. Pressman, Handbook of Cancer Immunology, H. Waters, Ed. (Garland STPM, New York, 1978) Vol. 5, pp. 29-50). However, the difficulty of obtaining reproducible, tumor-specific antibodies has markedly hindered advances in this area.
It has been demonstrated that monoclonal antibodies can give improved tumor location, partly because of the absence of interfering non-specific antibodies, and partly because of the high specificity and absence of cross-reactivity available in monoclonal antibodies (Ballou et al., Science 206, 844 (1979)). However, successful targeting using monoclonal antibodies in animals and humans has generally been disappointing. It is believed that the principal reason for such disappointing results has been the generally accepted assumption that monoclonal antibodies which are specific to tumors, and not to normal tissue, are required. Producing such antibodies has proved difficult and few, if any, have been found in spite of extensive research investigators in the area.
It has recently been demonstrated that absolute specificity to tumors is not necessary for proper tumor location (Ballou et al., J. Immunology 132, 2111-2116 (1984)). In that publication, a monoclonal antibody to a target present at much higher levels in normal tissues than in tumors was shown to localize specifically in tumors and not in the antigenic normal tissues.
In vitro and in vivo specificity are quite different as indicated by two different findings:
First, monoclonal antibodies which localize in tumors need not be truly tumor specific: examples of such antibodies are A2B5 (Reintgen et al., J. Surg. Oncol. 23, 205-211 (1983)) and anti-SSEA-1 (Ballou et al., J. Immunology, 132, 2111-2116 (1984)).
Second, antibodies which show a high level of in vitro specificity may not localize effectively. Mann et al., (Cancer 54, 1318, (1984)) developed monoclonal antibodies having an apparent specificity for each of two human tumors. The expectation was that, when the tumors were implanted in a living animal, and both antibodies were radiolabeled differently so as to enable them to be distinguished from each other, then each antibody would go only to the appropriate tumor. The results, however, showed that neither antibody localized to any appreciable extent.
Thus antibodies which target tumors selectively need not be specific, and antibodies which are apparently specific may not target.
In spite of these findings, tumor-localizing antibodies have been prepared by selecting monoclonal antibodies that are more highly bound to tumors than to normal tissues in in vitro screening assays. The present invention is for a novel selection methodology, which permits analysis of a wider range of antigens than are selectable by in vitro screening.